1. Field of the Invention
This invention relates to a new and improved method for illuminating a microscope slide and, in particular, illuminating a specimen in a darkfield.
2 Background
In many medical and biological research procedures, foreign particles and substances are used to label specific structures for the purpose of observation. For example, in autoradiography, biological specimens are covered with a photosensitive emulsion of silver grains to expose areas on which previously injected radioactive tags have collected.
One method of illuminating the specimens to show the silver grains is known as darkfield illumination. In this method, direct light is prevented from entering the objective by placing an opaque stop at the center of the condenser. This arrangement creates a light cone configuration permitting the observer to see the specimen illuminated by the light scattered or diffracted by it. Because the standard microscope condenser must be precisely covered with the opaque stop to form the light cone, using this darkfield illumination technique is time consuming. Most researchers replace the entire condensed with a condenser which has the opaque stop already fitted, but this too is time consuming. Also, each condenser is limited to a small range of magnifications, and, in fact, many darkfield condensers will not work with low power objectives.
Other disadvantages of this darkfield illumination technique include the difficulty of alignment as well as the requirement that the top lens of the condenser be immersed in oil if used with objectives of 0.80 numerical aperture or higher.
Another method of illuminating silver grains is the reflective polarized light method, also called EPI-polarized light. However, problems exist with this system at low magnification including the lack of sufficient light intensity as well as extraneous reflections which inhibit proper viewing. The reflective polarized light assembly also precludes the simultaneous use of EPI-fluorescence.